writer/msl: Add parentheses for member accesses

The LHS should be wrapped in parentheses if it has lower precedence
than the access. This fixes issues with pointer dereferences followed
by member accesses, where we were previously generating *a.b.

Fixed: tint:831
Change-Id: I8a194ad4f54c80a01c24eb983ec8064037575216
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/51963
Kokoro: Kokoro <noreply+kokoro@google.com>
Auto-Submit: James Price <jrprice@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>

src/writer/msl/generator_impl.cc

@@ -1728,9 +1728,20 @@ bool GeneratorImpl::EmitIf(ast::IfStatement* stmt) {
 }
 
 bool GeneratorImpl::EmitMemberAccessor(ast::MemberAccessorExpression* expr) {
+  bool paren_lhs =
+      !expr->structure()
+           ->IsAnyOf<ast::ArrayAccessorExpression, ast::CallExpression,
+                     ast::IdentifierExpression, ast::MemberAccessorExpression,
+                     ast::TypeConstructorExpression>();
+  if (paren_lhs) {
+    out_ << "(";
+  }
   if (!EmitExpression(expr->structure())) {
     return false;
   }
+  if (paren_lhs) {
+    out_ << ")";
+  }
 
   out_ << ".";
 

test/bug/tint/749.spvasm.expected.msl

@@ -1 +1 @@
-SKIP: crbug.com/tint/831
+SKIP: crbug.com/tint/833 loop emission is broken

test/ptr_ref/load/global/struct_field.spvasm.expected.msl

@@ -1 +1,16 @@
-SKIP: crbug.com/tint/831
+#include <metal_stdlib>
+
+using namespace metal;
+struct S {
+  int i;
+};
+
+kernel void tint_symbol() {
+  thread S tint_symbol_2 = {};
+  thread S* const tint_symbol_1 = &(tint_symbol_2);
+  int i = 0;
+  int const x_15 = (*(tint_symbol_1)).i;
+  i = x_15;
+  return;
+}
+

test/ptr_ref/load/global/struct_field.wgsl.expected.msl

@@ -1 +1,14 @@
-SKIP: crbug.com/tint/831
+#include <metal_stdlib>
+
+using namespace metal;
+struct S {
+  int i;
+};
+
+kernel void tint_symbol() {
+  thread S tint_symbol_2 = {};
+  thread S* const tint_symbol_1 = &(tint_symbol_2);
+  int const i = (*(tint_symbol_1)).i;
+  return;
+}
+

test/ptr_ref/store/global/struct_field.spvasm.expected.msl

@@ -1 +1,14 @@
-SKIP: crbug.com/tint/831
+#include <metal_stdlib>
+
+using namespace metal;
+struct S {
+  int i;
+};
+
+kernel void tint_symbol() {
+  thread S tint_symbol_2 = {};
+  thread S* const tint_symbol_1 = &(tint_symbol_2);
+  (*(tint_symbol_1)).i = 5;
+  return;
+}
+